Supply circuit for amplifiers



Aug. 8, 1933.

C. W. HANSELL SUPPLY CIRCUIT FOR AMPLIFIERS Filed May 19, 1950 Osczihfor 14 MpM INVENTOR CLARENCE W- HANSELL BY WM ATTORNEY Patented Aug. 8, 1933 SUPPLY CIRCUIT FOR AMPLIFIERS Clarence W. Hansell,

Port Jefferson, N. Y.,

assignor to Radio Corporation of America, a Corporation of Delaware Application May 19, 1930. 'Serial No. 453,465

4 Claims.

This invention relates to energy supply circuits for amplifiers and especially to a new and useful method and means for supplying anode potential to a high power amplifier.

In radio signaling systems it is common to use a rectifier, generator or other power supply source having a; considerable fluctuation or ripple in its voltage, for supplying anode potentials to amplifiers. In order to smooth out the unidirectional anode energy, condensers or filters containing condensers are placed across the output circuit of the power source which have very low impedances for the ripple energy and so cause currents to fiow through the internal impedance of the rectifier or generator of such value that the ripple voltage is substantially eliminated from the output.

The power sources used in practice also have a considerable amount of voltage regulation. In other words their output voltages decrease when the load current increases. The presence of the smoothing condensers slows up the change of voltage when the load current is changed. In a telegraph transmitter, where the load current fluctuates between zero and full load in accordance with the keying, the voltage at the beginning of each dot or dash is higher than the final value by the amount of the regulation. Consequently, when keyed energy is first applied to the input side of the amplifier there is often as much as a 25 percent overload voltage existing in the'amplifier relative to the steady marking voltage. Because of this, the peak electrical stresses on the amplifier tubes and circuits is greatly increased, tending to cause more rapid deterioration, and the total power output rating of the amplifier must be reduced to provide the extra safety factor demanded by the difference between the maximum voltage and'the steady load value. There is also a marked tendency for the vacuum tubes to develop parasitic oscillations duringv the amplifier, reduces to a predetermined value, the anode potential. For best operation I prefer to make this impedance'of a value such that when there is a tendency for momentary overloads, under the conditions outlined above, plate voltage is reduced to a value below that used for steady marking or steady key down position.

In order to increase the anode voltage during the remaining portion of the marking period, I shunt the resistance by a reactance, in the form of an inductance having low resistance which controls the rate of transfer of current from the resistance to itself and hence the voltage to the amplifier anode. However, when energy is removed from the amplifier input, the inductance tends to increase anode potential.

An increase at this time is less harmful than at the beginning of a signal because the radio frequency voltages are removed, but, it is undesirable, and in some cases can result in suificient increase in voltage to be destructive. This presents a disadvantage in that, by virtue of the added potential, stresses in the tubes and circuits are increased. Therefore, the overcoming of this defect is a further object of my invention and I do this by utilizing the resistance which produces initial reduced anode voltages to resistively dissipate the inductive potential surge following the removal of the load on the power source resulting from the removal of the energy from the input circuit of the amplifier. Consequently the impedance formed of the resistance and inductance prevents momentary overloads on the amplifier tubes when signaling energy is first introduced therein and prevents excessive plate voltages when signaling energy is removed from the input circuit of the amplifier.

In connection with the use of the resistance, it is to be noted that it presents other advantages in that it lessens relatively slow oscillations in plate voltage caused by the cyclic charge and discharge of the smoothing condensers, connected across the rectifier output circuit, through the'inductance of the power source and filter circuits. In other words it damps out resonance efiects in the power circuits which may cause large voltage fluctuations at certain keying speeds or modulating frequencies.

My invention is .defined with particularity in the appended claims; however, it may best be understood both as to its method of operation and structural organization by referring to the accompanying drawing, in which Figure 1 illustrates a transmitting system incorporating the features of my invention, and

Figures 2 to 7 inclusive are given by way of explanation thereof.

Referring to Figure 1, relatively low frequency alternating current supplied by source 2 is keyed or otherwise modulated at some centrally located point 4, and transmitted over land line 6 to radio transmitting apparatus.

The received keyed energy is rectified in a rectifier 8 the output of which is used to control the output of a high frequency oscillator 10, for example, of the crystal controlled variety, or of one of the radio frequency amplifiers or frequency multipliers between a crystal controlled amplifier and the power amplifier. The keyed output of the oscillator or amplifier may then be fed into suitable frequency multipliers and am-'- plifiers 12 which in turn supply their energy to the tuned input circuit 14 of 'a power amplifier 16 having a tuned output circuit 18, anodes 20, control electrodes 22 and cathodes 24. Output of the power amplifier 16 is fed from its output circuit 18 to, for example, an antenna 26 and transmitted to a receiving station (not shown).

Anode potential for the power amplifier, is derived from a source of alternating current 28 coupled through transformer30 torectifier 32. For smoothing purposes condensers are connected across the output circuit of the rectifier and, for the sake of simplicity, they have all been represented as a dotted capacity 34. The lead 36 between the rectifier and the anodes of the power amplifier supply the anodes of the amplifier with positive potential.

Because of the inherent re ulation of the rectifier, in the absence of keyed energy in the input circuit 14 of high power amplifier 16, the voltage applied to the anodes of the amplifier is given by the value 36 of Figure 2. During steady key down position, the anode potential is represented by value 38. Now, at theinstant when keyed energy is first introduced into the power amplifier, capacity 34 tends to maintain high voltage on the anodes resulting in the overload peak 40 of Figure 3 which indicates the excessive plate current flowing at that time. Shortly thereafter this current drops to its normal value 42, but during the short time that current fiows as at point 40 of Figure 3, the tubes and circuits are subjected to abnormally high stresses and loads. Not only is this poor from the apparatus point of view but from the aspect of signaling the increased current introduces at point 42 an increased antenna current giving rise to undesirable energy modulation which appears as a click in the receivers.

To overcome the foregoing difficulty' I insert in series with the anode lead 36 a filter or impedance composed of inductance 44 and resistance 50 which reduces plate voltage to a value 46 (Figure 5) much below the normal plate voltage 38 representing the normal marking plate potential. In this manner, plate current fiow is as shown in Figure 6, and antenna current as indicated in Figure '7, free from peaks and their attendant evils.

The value of resistance 50 is so adjusted that, at the instant the load is thrown on, the voltage drop in the resistance will give the desired maximum reduction of voltage on the anodes of the amplifier. It is to be noted that, at the first instant, no current flows in the inductance 44 because a potential must be applied for a finite time to build up a current in an inductance.

Having arrived at the desired value of resistance 50 the inductance 44 is adjusted to give the desired rate of increase on the anodes of the amplifiers from the minimum value up to the normal steady load value. If it is desired to have the signal as nearly square as possible then the time constant of the inductance 44 and resistance 50 is made substantially the same as the time constant of discharge from the smoothing condenser under the conditions imposed. In practice some of the radio frequency energy output from the amplifier is rectified and observed with an oscillograph. The values of inductance 44 and resistance 50 are then adjusted for the desired signal shape. Usually the building up'of the signal is slowed down as much as can be permitted by the highest signaling speed which will be used. After the values are determined for one transmitter they may be simply duplicated for similar transmitters. The resistance offers an additional advantage in that if there is any tendency for the internal inductance of the rectifier transformers to introduce surges of potential on plates 30 of amplifier 16, the energy causing the surges will be dissipated by it. As previously explained the resistance also absorbs the surge in inductance 44 when the load current is interrupted.

I claim:

1. In combination, a power amplifier having a high frequency anode circuit and an input circuit; a source of rectified alternating current for supplying the anode with unidirectional potential; means for supplying keyed input energy to the input circuit of the amplifier; and a circuit, comprising the parallel connection of an inductance and. a resistance, in series with the anode supply lead from the rectifier and between the anode circuit and rectifier for reducing anode potential when keyed energyis first introduced into the input circuit of said amplifier.

2. In a signaling system, means for keying a source of low frequency energy; a rectifier for rectifying the keyed energy; a high frequency oscillator responsive to the rectified energy; a power amplifier coupled to the high frequency oscillator; a rectifier having condensers connected across its output circuit rectifying alternating current supplied to it for producing unithe power amplifier is removed and for eliminating transient oscillations in the anode circuit of the amplifier; and, a high frequency output circuit coupled to the anodes of the amplifier and connected between the anodes and said inductance and resistance parallel combination.

3. In a power amplifier, an electron discharge device having an anode a cathode and a control electrode, a high frequency circuit connected between said control electrode and cathode, a high frequency circuit connected to said anode, a source of anode potential, and, the parallel c'om bination of an inductance coil and a resistance connected between said high frequency anode circuit and said source of, potential, said parallel combination of coil and resistance acting to reduce overloads on said electron discharge device when energy is first introduced into said input circuit, by causing the anode current to flow through said resistance and subsequently transferring the anode current from said resistance to said inductance, which inductance in turn is of low resistance and determines the rate of transfer of current from said resistance to said inductance.

4. In a signaling system, a power amplifier comprising a pair of pushpull connected electron discharge devices, a parallel tuned circuit connected to the anodes of said electron discharge devices, a source of anode potential, and a con- 

